well leonardo
You suppose that all people are in good faith, most of people of course they are
but not all
Suppose that you buy some models for a few dollars then your game is a tremendous hit
It can happen, not so often but it happened
The owner of the models sue you asking for royalties
My question is
Should you ask also for a written contract or a PayPal payment with the wording " models for a commercial game " or something like that, is sufficient to protect your interest